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Lesson Plan #5 http://www.phy6.org/Stargaze/Lseason.htm

(3) Seasons of the Year

The link between the tilt of the Earth's axis to the
ecliptic and seasons of the year--length of day, effectiveness of sunlight, polar day and night, and seasons south and north of the equator, as well as near it.

Part of a high school course on astronomy, Newtonian mechanics and spaceflight
by David P. Stern

Know how the inclination of the Earth's rotation axis--the 23.5 degree angle between it and the perpendicular to the ecliptic--causes seasons of the year, by varying the length of the local day and the angle at which the Sun's rays arrive on the surface of the Earth.

Recognize that seasons in the southern hemisphere (at middle latitudes) occur at opposite parts of the year from seasons in the US and Europe.

Realize that the seasons in the midle-latitudes in US and Europe are quite different from seasons experienced near the equator.

Know about the polar day, when the Sun never sets but just marches around the horizon, and the polar night, when it never rises (though one may get "white nights" of twilight, when the Sun is just below the horizon).

Terms: Inclination of the Earth's axis, polar night, polar day, solstice, equinox--also summer solstice (on or near June 21), winter solstice (on or near December 21), spring (or vernal) equinox (on or near March 21) and fall (or autumnal) equinox (on or near September 22).

Stories and extras: The oceans moderate and delay summer and winter. Earth is closest to the Sun in early January.

Start by asking the class: who knows why summers are hotter than winters? (Two reasons--days are longer, and the Sun's rays arrive more steeply,) We are not closer to the Sun in summer--otherwise, why would summer in the US be winter in Argentina and Chile?. Actually, Earth is most distant from the Sun in summer!

And by the way: Shakespeare wrote a delightful play "Midsummer Night's Dream." When is "midsummer night"? What is so special about it?

(It is the shortest night of the year, about June 21. In Baltic and Nordic countries this is often a time for celebration.)

After this, present the material. The questions below may be used in the presentation, the review afterwards or in both

-- If the rotation axis of the Earth were perpendicular to the ecliptic, would day and night be equally long?

Yes

-- The actual rotational axis of the Earth is not perpendicular to the ecliptic, but makes an angle i with the perpendicular direction. How big is the angle i?

i = 23.5 degrees

--Because of the inclination angle i, sometimes the North Pole of the Earth leans towards the Sun, sometimes it leans away. How are those times connected to the seasons of the year?

The northern half of Earth leans towards the Sun when the northern hemisphere has summer, away from the Sun when it has winter.

-- Why does winter in the northern hemisphere (e.g. in the US) correspond to summer in the southern hemisphere (e.g. Argentina), and vice versa?

Because when the North Pole leans towards the Sun (producing summer north of the equator), the south pole automatically leans away from it (producing winter south of the equator)--and vice versa [demonstrate with a globe, a ball or with fruit]

-- When it is summer north of the equator, how does the Sun appear from the North Pole? What about points near the North Pole?

In northern summer, the pole is always in sunlight, 24 hours in a day. The same holds for points near the North Pole for an appreciable part of summer. This area defines the Arctic Circle.

-- In summer, at or near the North Pole--how does the Sun appear to move?

It seems to be moving around the horizon.

[The North Pole is not a good choice for a science base, because it is in mid-ocean, covered with floating sea ice. However, the South Pole is located on the thick Antarctic ice cap and in fact a large scientific base is located there. In the middle of southern summer (e.g. December), that is a good place for observing the Sun, and the science station does in fact include a solar observatory. One reason for it is the stability of the cold atmosphere (the image does not jitter), but another is that in mid-summer it can observe the Sun continuously for weeks at a time!]

--In winter, at or near the North Pole, how does the Sun appear?

It does not appear, it never rises. The location is in continuing darkness--the "polar night."

--How does the length of the day change with season?

In or near the pole, during mid-summer, the days are 24 hours long. In mid-winter, they are zero hours long. At lower latitudes, summer days are longer while winter days are shorter.

--What are the solstices and when do they occur?

Solstices are the times when the Earth axis leans directly in the direction to the Sun--one pole leans sunward, the other leans away from the Sun. North of the equator, midsummer day is around June 21. Midwinter day falls around December 21. South of the equator it is the other way around. Dates may vary by a day or so).

--When is the shortest day, and when is the shortest night?

The shortest day is the day of the winter solstice December 21, in the US. The shortest night is the day of the summer solstice (June 21).

-- What are equinoxes, and when do they occur?

Equinoxes are the times in the Earth's orbit when the lean of the Earth's axis is neither towards the Sun nor away from it, but exactly sideways [demonstrate with a globe, a ball or an apple]. The axis of the Earth is then perpendicular to the direction of the Sun.

Show that the dividing line between light (=day) and darkness (=night) then runs from pole to pole (and back to the first pole).

The equinoxes occur around March 21 ("spring equinox" or "vernal equinox") and September 22 ("fall equinox" or "autumnal equinox.").

-- What is the reason for the name "equinox"?

Night ("nox") is about equal in length to the day.

-- If June 21 is the longest day of the year, why isn't it also the warmest?

The oceans are slow to heat up and cool resulting in a moderating influence over temperature variations.

-- When is Earth closest, in its orbit, to the Sun?

The Earth is closest to the Sun between January 3 and 5. The difference is a few percent, too small to be seen.

(Don't bring up the following question--it is too distracting--unless some student asks it. If you count the days from the fall equinox to the spring equinox, the (northern) winter half of the year, you find its one day longer than half the year, while the summer half is one day shorter. Why?

The answer is discussed in section #12a, and has to do with the fact that the orbit of the Earth around the Sun is not a circle but an ellipse, it is slightly elongated. In winter we are closer to the Sun, in the "smaller half" of the ellipse, and also, Earth then moves a little faster. Both effects make the "winter half" of the year (northern winter) slightly shorter.)

--Strictly speaking, those above distances are from the center of the Earth to the center of the Sun. Actually, in summer when the northern hemisphere is inclined towards the Sun (the teacher should draw this schematically on the board), the northern hemisphere is somewhat closer to the Sun than the southern hemisphere, which has winter at that time. Could that be the reason why summers are hot and winters cold?

(Some students may make guesses)

Let us calculate. The Sun is about 23500 Earth radii away from Earth. So....?

(Students should realize from the drawing on the board, that the difference in the distance of the northern and southern hemisphere is about 1 Earth radius or less: much smaller than the difference in the Sun's distance due to the ellipticity of the Earth's orbit.

So that ellipticity should have a bigger effect on the heat we receive. Yet it clearly isn't the cause of the seasons; if it was, the northern and southern hemispheres would be heated or chilled at the same time!)